Kiln plant control system

ABSTRACT

A controller for a kiln plant, typically a cement plant, has a thermodynamic controller which measures a number of variables including the kiln hood temperature and one or more output gas concentrations, and controls the fuel input to the kiln to maintain the hood temperature within a desired range and a main impeller of the kiln to maintain the measured gas concentrations within a predetermined range. The invention includes a quality controller wich controls the amount of 3CaO.SiO2 present in clinker produced by the plant. The controller comprises an inner controller which controls free-lime content in the clinker and a thermodynamic outer controller.

BACKGROUND OF THE INVENTION

This invention relates to a method of controlling a kiln plant and to asystem for implementing the method.

In the manufacturing of cement, a kiln plant is used to convert raw mealto clinker which is then milled together with other materials to producecement. Due to the large number of variables which affect the operationof the kiln plant and the quality of the clinker, various controlsystems and methods have been proposed over the years. Nevertheless, itremains difficult to optimise the operation of such a plant,particularly due to variations in the feed material, fuel quality,ambient conditions and other variables.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided acontroller for a kiln plant, the controller comprising:

a first temperature sensor arranged to measure the temperature at ornear the hood of the kiln and to generate an output indicative of thistemperature;

gas sensing means arranged to measure the concentration in the kiln ofat least one gas from the group comprising O₂, NOX, SOX and CO and togenerate at least one respective output indicative of the relevant gasconcentration; and

control means adapted to receive the output from the first temperaturesensor and to control the amount of fuel fed to the firing end of thekiln to maintain the temperature at or near the hood of the kiln withina predetermined range, the control means being further adapted toreceive said at least one output from the gas sensing means and tocontrol at least a main impeller of the kiln to maintain theconcentration of said at least one gas within a predetermined range.

The controller preferably further comprises a second temperature sensorarranged to measure the temperature at or near the back end of the kilnand to generate an output indicative of this temperature, the controlmeans being further adapted to receive the output from the secondtemperature sensor and to control the amount of fuel fed to the back endof the kiln to maintain the temperature at or near the back end of thekiln within a predetermined range.

The control means may further include a control matrix which includesvalues determinative of the relationships between a plurality of plantmeasurements including the temperature at or near the hood of the kiln,the temperature at or near the back end of the kiln, and O₂, NOX, SOXand CO concentrations, and a plurality of operating parameters includingthe amount of fuel fed to the firing end of the kiln, the amount of fuelfed to the back end of the kiln, the main impeller speed, the kilnspeed, the kiln main drive current, the raw meal feed, the cooler airflow and cooler grate speed.

The invention also extends to a method of controlling a kiln plant usingthe controller described above.

According to a second aspect of the invention there is provided controlmeans for a kiln plant, the control means comprising an outer qualitycontroller cascaded to at least one inner controller, wherein the outerquality controller comprises a first feedback controller being adaptedto receive a first setpoint input indicating a desired amount of3CaO.SiO₂ and/or 2CaO.SiO₂ and/or another clinker chemical property tobe present in clinker produced by the kiln plant, and a second feedbackinput indicating the actual amount of 3CaO.SiO₂ and/or 2CaO.SiO₂ and/oranother clinker chemical property present in clinker being produced bythe kiln plant, the first feedback controller being further adapted tocompare the first setpoint input and the second input and, if the inputsdiffer, to produce an output to alter a setpoint input to the innercontroller directly or indirectly to adjust one or more of the kilnplant's operating parameters so that the amount of 3CaO.SiO₂ and/or2CaO.SiO₂ and/or another clinker chemical property in the clinkerproduced by the kiln plant will be substantially equal to the desiredamount of 3CaO.SiO₂ and/or 2CaO.SiO₂ and/or another clinker chemicalproperty.

Preferably, the inner controller is a free lime controller, wherein thesecond setpoint input comprises a dynamic setpoint for the free limecontent of the clinker to the inner controller, and wherein the innercontroller is adapted to receive an input indicating the actual amountof free lime present in clinker being produced by the kiln plant, theinner controller being further adapted to compare the dynamic setpointfor the free lime content and the actual amount of free lime presentand, if these differ, to produce an output to directly or indirectlyalter one or more of the kiln plant's operating parameters so that theamount of free lime present in the clinker produced by the kiln plantwill be substantially equal to the dynamic setpoint for the free limecontent.

In a first embodiment of the second aspect of the invention, the controlmeans may still further include a thermodynamic controller, cascaded tothe inner controller, wherein the inner controller outputs a setpointfor at least one plant measurement to the thermodynamic controller, andwherein the thermodynamic controller is adapted to receive an input fromthe kiln plant indicating the value of the at least one plantmeasurement, the thermodynamic controller being further adapted tocompare the setpoint for the at least one plant measurement and thevalue of the at least one plant measurement and, if these differ, toproduce an output to alter one or more of the kiln plant's operatingparameters.

Preferably, the inner controller is arranged to output a plurality ofdynamic setpoints for a plurality of plant measurements to thethermodynamic controller, the plurality of plant measurementsconstituting controlled variables and being selected from the groupincluding the back end temperature, the hood temperature, the level ofCO, the level of NOX, the level of SOX and the level of O₂.

The kiln plant's operating parameters may comprise one or more of thegroup constituting manipulated variables comprising the total fuel fedto the kiln plant, the percentage fuel fed to the back of the kiln plantor any other derived measurement or indication of the fuel being fed tothe plant, the main impeller speed, the kiln speed, the cooler air flowand the cooler grate speed. These parameters are manipulated to alterthe plant measurements to approach respective setpoints, using a controlmatrix which includes values determinative of the relationships betweenthe operating parameters and plant measurements.

In a second embodiment of the second aspect of the invention, thecontrol means may still further include a thermodynamic controllerconnected to the kiln plant, wherein the thermodynamic controller isadapted to receive an input from the kiln plant indicating the value ofat least one plant measurement, the controller being further adapted tocompare a setpoint for the at least one plant measurement and the valueof the at least one plant measurement and, if these differ, producing anoutput to alter one or more of the kiln plant's operating parameters,wherein the at least one of the kiln plant's operating parameterscontrolled by the thermodynamic controller is different from the one ormore operating parameters controlled by the free lime controller.

The at least one or more of the kiln plant's operating parameterscontrolled by the thermodynamic controller comprise at least one of thegroup comprising the total coal fed to the kiln, the main impellerspeed, the kiln speed, the raw meal feed, the cooler air flow and thecooler grate speed and wherein the kiln plant's operating parametercontrolled by the free lime controller is the percentage fuel fed to theback of the kiln.

In a third embodiment of the second aspect of the invention, the innercontroller is a thermodynamic controller, wherein the second setpointinput from the outer quality controller to the thermodynamic controlleris a setpoint for at least one plant measurement, and wherein thethermodynamic controller is adapted to receive an input from the kilnplant indicating the value of the at least one plant measurement, thethermodynamic controller being further adapted to compare the setpointfor the at least one plant measurement and the input indicating thevalue of the at least one plant measurement and, if these differ, toproduce an output to alter one or more of the kiln plant's operatingparameters.

The at least one plant measurement may be one or more of the plantmeasurements selected from the group including the back end temperature,the hood temperature and the level of NOX.

Preferably, the at least one plant measurement is the hood temperature.

In this embodiment, the control means also includes a free-limecontroller arranged to receive a setpoint input for the free-limecontent of the clinker and an input indicating the actual amount offree-lime present in clinker being produced by the kiln plant, thefree-lime controller being further adapted to compare the setpoint forthe free-lime content and the input indicating the actual amount offree-lime present and, if these differ, to produce an output to directlyor indirectly alter one or more of the kiln plant's operating parametersso that the amount of free-lime present in the clinker produced by thekiln plant will be substantially equal to the setpoint for the free-limecontent.

The setpoint for the free-lime may be received from the 3CaO.SiO₂controller, or may be manually inputted by an operator of thecontroller.

The one or more operating parameters controlled by the free-limecontroller are different from the one or more operating parameterscontrolled by the thermodynamic controller.

Preferably, the operating parameter controlled by the free-limecontroller is the percentage fuel fed to the back of the kiln.

The invention also extends to a method of controlling a kiln plant usingthe controller described above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram showing the basic arrangement of acement production plant;

FIG. 2 is a more detailed schematic block diagram of the cementproduction plant, showing the kiln plant thereof in more detail;

FIG. 3 is a simplified block diagram illustrating the basic steps incement production;

FIG. 4 is a graph showing a typical temperature profile along the kilnplant;

FIG. 5 is a schematic diagram illustrating the source of the variousmonitored variables utilised by the controller;

FIG. 6 is a matrix illustrating the relationship between the variablesutilised by the controller.

FIG. 7 is a simplified block diagram of the existing control system of acement plant used to test the method and system of the invention;

FIG. 8 is a simplified schematic diagram illustrating the integration ofthe controller of the invention with the control system of FIG. 7;

FIG. 9 is a schematic block diagram showing a first embodiment of thecontrol loop arrangement of the controller of the invention;

FIG. 10 is a schematic block diagram showing the relationship between3CaO.SiO₂ and free lime;

FIG. 11 is a schematic block diagram showing a second embodiment of thecontrol loop arrangement of the controller of the invention; and

FIG. 12 is a schematic block diagram showing a third embodiment of thecontrol loop arrangement of the controller of the invention.

DESCRIPTION OF EMBODIMENTS

FIG. 1 shows, in a simplified block diagram form, the basic arrangementof a cement production plant.

The core of the cement plant is the kiln plant, the primary piece ofequipment of which is the kiln itself. The kiln is the primary processunit in which clinker is made. The kiln plant comprises the kiln as wellas a number of additional up-stream and down-stream process units. Thenumber of these additional units surrounding the kiln depends on thetype, design and age of the cement plant. FIG. 2 shows a typical kilnplant 10 including a kiln 12 and associated equipment, described in moredetail below.

Cement is made primarily from clinker (>75%) and other constituents,such as gypsum and various other extenders. The cement plant uses as itsraw materials inputs, limestone, and various other minerals, such asclay, shale, and materials containing iron oxide. The input materialsare crushed, screened, milled, and mixed to form raw meal, which is theprimary feed to the kiln plant. The kiln plant produces clinker, whichis then mixed and milled with gypsum or other extenders such as slag toform cement. The overall process is shown in FIG. 1. The method andsystem of the invention controls the kiln plant only, i.e. thetransformation of raw meal into clinker, and not the entire cementplant.

The process that transforms raw meal to clinker in the kiln plant iscalled “burning” or sintering and is basically a multistage processwhich primarily uses hot air to effect the necessary chemical andmetallurgical transformations in the raw meal. Most of the processtransformations take place in the kiln 12 which is typically a slowlyrotating steel tube 3 to 5 m in diameter and with a length ranging from50 to 250 m and typically around 80 m. The tube is inclined at a slightangle to the horizontal. The raw meal moves through the kiln by virtueof the rotation of the tube, whereby material slowly shifts or slidesdown the kiln due to its incline to the horizontal.

On older or simpler kilns all transformations take place in the kiln. Inmore modern plants, the first part of the process typically takes placein an upstream unit called a pre-heater or pre-calciner 14, wherepreheating, drying, and calcining takes place. This process is shown inFIG. 3. The actual clinkering process as well as some calcining thentakes place in the kiln. The objective of splitting up the process intothe pre-heater/calciner plus kiln configuration is to achieve betterheat efficiencies and to vent less heat to waste. The cement plant towhich the prototype method and system of the invention were applied usesa pre-heater.

The following description applies to the prototype kiln plant and itwill be appreciated that various changes may be required to implementthe invention in a kiln which operates differently.

Referring to FIG. 2, the raw meal which is used as the feed material tothe kiln is obtained by mixing raw materials from stock piles 30 in aproportioning plant 32, from where they are fed to a raw mill 34 whichcomminutes the raw meal. The raw meal is stored in a raw meal blendingor homogenizing silo 36 and fed by means of a raw meal feeder conveyor38 in the kiln plant 10 to the pre-heater/pre-calciner 14.

In general and irrespective of the physical arrangement of the kilnplant, the first process is the drying and heating of the raw meal toapproximately 800° C. to 1000° C., where all moisture, both free andinherent in the raw meal, is driven off. In addition, certain chemicalreactions start to take place where for example the calcium carbonateand any magnesium carbonate in the raw meal are split up into calciumand magnesium oxides and carbon dioxide which leaves with the gases(referred to as “calcining”). The reactions involved are endothermic. Inthe case of a pre-heater configured plant, this calcined or partiallycalcined material is then fed into the kiln at the “feed end”. Thismaterial slowly slides down the kiln towards a flame located at theother end of the kiln called the “firing end”. This process can alsotake place in a precalciner placed in parallel to the pre-heaters.

As the temperature increases, the alumina and iron oxides in thematerial start to react with the calcium oxide to form calciumaluminates, mainly C₃A, and calcium alumino ferrites, typically C₄AF.These materials have a relatively low melting point, and form a melt orliquid in the material mix called a flux. The presence of this meltassists in bringing solid calcium oxide particles and solid silica andsilicate particles together to react to form calcium silicates.Initially all silica is converted to di-calcium silicate (2CaO.SiO₂),and to tri-calcium silicate (3CaO.SiO₂). The latter process is primarilyan exothermic processes.

The final silicate reactions take place in the burning zone of the kilnwhere the flame is located and where the temperatures are approximately1400-1500° C. The reactions take place with the material in a partialmelt form, which is sometimes called sintering.

The clinker that exits the kiln is very hot, approximately 1300° C., andneeds to be cooled down to ambient temperature. This is done in a coolerunit 16 which utilizes one or more fans 18 and which is downstream fromthe kiln. The cooler in the prototype plant was a grate cooler. Variousother types of coolers can be used, for example planetary coolers.Planetary coolers do not have separate fans like grate coolers.

The heating of the kiln is performed by injecting coal or other fuels(e.g. oil, tyres, gas, waste materials) into the kiln and then ignitingthis fuel to create heat from a long flame. The fuel ignition becomesself-sustaining once the kiln has reached a sufficiently hightemperature. The injection of fuel can be done at both ends of the kiln,but the primary point of coal (fuel) input is at the “fire-end” of thekiln. In order to keep the combustion process going, air must be suckedthrough the kiln to provide oxygen. This is done by a main impeller inthe form of an induced draft fan (ID fan) 20 which is connected to thekiln via ducts and piping. If a pre-heater or pre-calciner is used, thenthese units are situated between the ID fan and the kiln, so that theair is drawn through the kiln, then the pre-heater/pre-calciner, andfinally through the ID fan itself. After exiting the ID fan the air ispassed through a dedusting system such as a static precipitator 22 andthen vented to atmosphere via a stack 24.

The air drawn into the kiln comes from two main sources, the primary airbeing air injected by a primary fan 26 with the coal which is beingpneumatically conveyed from a coal plant 28 and injected into the kiln,and the secondary air which is drawn from the cooler 16. The air fromthe cooler is hot due to the heat exchange process that takes placebetween the hot clinker being fed into the cooler, and the cooler air(ambient temperature) being drawn or blown in the cooler.

The cooler can take various forms but the cooler used for testing thisinvention was a grate cooler, which has a number of fans pumping ambientair into the cooler.

It will be understood from the above description that there is acounter-current flow of air to the flow of material in the kiln plant.This airflow is thus as follows: air enters the cooler, which is heatedby the hot clinker exiting the kiln. This heated air, called “secondaryair”, is then fed into the fire-end of the kiln where it aids theheating and combusting process in the kiln. This heated air becomeshotter, and then exits the kiln where, if a pre-heater, or pre-calcineris used, it is fed into these units to affect the calcining and dryingprocess therein.

The hot air exiting the kiln is used in the pre-heater and/orpre-calciner to pre-calcine, heat and dry the raw meal, where throughthe heat exchange process in the pre-heater the air becomesprogressively cooler. The air exits the pre-heater at approximately 300°C. This exit air is then possibly used elsewhere in the plant to drymaterial in the raw meal or coal plants before ultimately being fed intoa dedusting system and then being vented to the atmosphere. With plantswith no pre-heaters or pre-calciners, heat exchangers in the form ofcrosses/chains/lifters which are usually in the form of steel plates orsteel chains fitted into the inside of the feed end of the kiln, areused to effect this heat exchange. This exit air also exits atapproximately 300° C. before being fed into the dedusting system.

In general, operators control the kiln plant by adjusting the behaviorof a number of units or pieces of equipment in the plant in response tothe plant states. Knowledge of the plant states is inferred by theoperators from information being displayed by the plant supervisory,control and data acquisition system (SCADA), distributed control system(DCS), or other forms of electrical instrumentation.

Information is fed to the SCADA system by various sensors installedthroughout the plant. These sensors provide information on the state ofvarious pieces of equipment, as well as the state of the processestaking place. Essentially the SCADA system provides a view or “window”into the kiln plant, by displaying data in the form of icons, tables,graphs or plots. The SCADA system also provides the means with which theoperators change or control the various equipment, by switchingequipment on or off, starting or stopping sequences, or providingsetpoints to various pieces of equipment.

The operators have a number of control tasks to perform, these being thecontrol of the plant from a mechanical and safety point of view, as wellas the control of the plant in terms of production and quality.

However, it will be appreciated that the various sources of heat in thekiln, namely from the flame located in the kiln hood, from theexothermic chemical reactions and the heat retrieved from the coolermake the thermodynamics of the kiln very difficult to control.

Thermodynamic control of the kiln plant is the control of the kiln plantso as try to maintain the efficient production and quality of theclinker as discussed above. Thus the primary objective of thermodynamiccontrol is to continuously produce clinker meeting the qualityspecifications, whilst at the same time reducing production coststhrough the minimization of fuel consumption and stresses to the plantmechanically, in particular the kiln refractories. This purpose isachieved by controlling the thermodynamic state of the kiln plant. Thekiln plant includes the cooler, kiln, pre-heater and, if it exists, thepre-calciner. When the thermodynamic controller is controlling thethermodynamic state or profile of the kiln plant, then other higherlevel objectives of the total controller can be met, i.e. the quality ofthe clinker can be controlled.

A first aspect of the present invention is therefore the provision ofcontrol means to control the thermodynamics of the kiln plant.

Thermodynamic control of the kiln plant is addressed by observing theplant states through sensor measurements, and then adjusting the plantunit's behavior so as to maintain the correct thermodynamic conditionsto in turn effect the correct calcining and clinkering process. At thesame time, maximum production must be maintained, and the various costsminimised.

The most direct indication of the thermodynamic state of the kiln plantis the temperature profile along the kiln plant, which includes the kilnand upstream and downstream units to the kiln i.e. the cooler,pre-heaters and pre-calciners. This temperature profile can range from asingle point to a multi-point indication of the profile. A simplistictemperature profile (not to scale) is illustrated in FIG. 4.

There are in general too many factors for an operator to take intoaccount at all times when attempting to control the kiln plant manually,and optimization is thus simply not possible to achieve manually.Eventually some acceptable steady state performance is reached, wherethe kiln is performing adequately in terms of throughput and quality.This is relative to today's standard of kiln operation, which is basedon manual or partially automated control.

The difficulty in manual control is partly due to the difficulty inascertaining what is going on inside the kiln, from both a process and ametallurgical point of view. This is a universal problem, which isespecially accentuated in high temperature systems where directcontinuous industrial temperature measurement does not exist. This notonly includes the actual thermodynamic conditions, but the actualmetallurgical and physical states of the material being converted intoclinker, as well as the state of the coating that adheres to therefractory bricks of the kiln. It is currently assumed that theinstrumentation around the kiln is sufficient to allow for valid andcorrect assumptions to be made about the thermodynamic and clinkerproduction process in the kiln.

Furthermore, laboratory measurements are in general labour intensive andthese quality measurements are only available approximately one hourafter the sample was taken. At the plant used to test the invention thesample collection and most analysis is automated, in addition to manualsampling and analysis. Laboratory analysis is also performed on the rawmeal feed to the kiln, as well as on the coal being used for heating.Again these measurements are only available some time after the samplesare taken.

In addition, changing the thermodynamic conditions in the kiln tocompensate for the source of the problem also takes time. Compoundingthis is the fact that the results of some of the control actions takeseconds to manifest but in other cases take 20 minutes to an hour. Inparticular, changes to the gas or air state of the kiln manifestsrelatively rapidly (order of seconds), while changes to thethermodynamic or temperature state manifest relatively slowly (order ofminutes), and changes to the quality or metallurgical state, i.e.free-lime and 3CaO.SiO₂, manifest more slowly still (order of an hour).

Compounding this further is the fact that there is a delay before atemperature change in one part of the system, e.g. in the cooler orkiln, has an effect which is noted in the pre-heater, and vice versa. Inaddition there are intrinsic delays caused by flow of material in thekiln and cooler.

The time delays thus further complicate the control of the kiln plant,in that the operators tend to take action too late to compensate fordisturbances.

The kiln experiences a number of major disturbances during operation.These disturbances include:

(1) “poking” in which inspection doors are opened in the feed-end riserpipe on the bottom of the pre-heaters, or other parts of thepre-heaters, and cold jets of air and/or water are used to introducepneumatic and thermal shock to deposits and build-up of materials whichhave to be cleared to enable continuous and free flow of partiallycalcined material from the pre-heaters in the kiln.

(2) “coating drop” where part of the clinker coating in the kiln fallsoff the inner linings or refractory bricks in the kiln, causing anunexpected increase in the flow of very hot material into the kiln.

Both these types of disturbances cause great thermal disturbances in thekiln plant as a whole, and are very difficult to control manually. Theytherefore usually result in a large of amount of off-specificationmaterial being produced, as well as thermodynamic disturbances which canlast for up to a few hours.

It will thus be appreciated that all of the above factors taken togethermake the efficient control of the kiln very difficult to achievemanually or using the partially automated controllers available atpresent.

In a first aspect of the present invention, a new type of kilncontroller has been implemented which focuses on controlling thethermodynamics of the kiln in order to effectively control the qualityand throughput of the kiln.

Referring again to FIG. 4, the temperature profile along the kiln isdetermined from temperature sensors which are ideally arranged atintervals from the cooler to the pre-heater along the physical length ofthe kiln plant. The sensors will typically be thermocouples, but may beany other type of temperature sensor, such as pyrometers.

The measured temperatures can also be derived using, for example, anaverage or another kind of mathematically derived or filteredtemperature which indicates the thermodynamic state of the kiln in thevicinity of the kiln hood or wherever the measurement is being made.

It is preferable to have more sensors available to provide a moreprecise temperature profile. However, if the precise temperature profileor shape is not known, the control of the kiln can be limited to thenumber of thermodynamic control handles available.

Where only one sensor is used, this sensor will measure the hoodtemperature in the kiln, which is the most important temperaturemeasurement. However, the preferred number of sensors is at least two,giving two degrees of freedom in terms of controlling the temperatureprofile. For a two point temperature profile, another temperaturemeasurement is required at the feed end of the kiln plant, such as thekiln back end or feed end. If a pre-heater system exists, a sensor maybe placed at some point in the pre-heater system, or at the top of thepre-heater.

To achieve thermodynamic control of the plant, the sources and sinks ofenergy within the plant must be controlled so that there exists anoptimal heat balance in the kiln, which will in turn require the minimumamount of energy to effect correct clinker production that meets minimumquality requirements. Thus the coal feed input energy is minimized, therecovery of secondary heat from the coolers is maximized and, with agrate cooler, a minimum amount of energy or heat is vented from thebackend of the cooler through the cooler ID-fan to atmosphere.

The heat and hence temperature profile in the kiln plant is controlled,according to the present invention, using a multivariable thermodynamiccontroller, which has a number of controlled variables as well as anumber of manipulated variables. The controlled variables are usually anumber of process variables that give information and knowledge of thestate of the kiln.

In the prototype system, the thermodynamic controller was implemented asa 6×5 control matrix, i.e. 6×5 single input, single output (SISO)control pairs, where each pair or matrix element consists of acontrolled variable (CV) 52, and a manipulated or disturbance variable(MV, or DV) 53. FIG. 5 shows schematically the source of the relevantvariables. The control matrix is illustrated in FIG. 6.

The controlled and manipulated variables configured on the test plantwere:

Controlled Variables (CVs)

(1) Hood temperature (THood). The hood temperature is measured near thefiring end of the kiln, and is indicative of the temperature of the airetc, near the firing end of the kiln. This temperature may be the exactvalue as measured, or can be a derived value from a number oftemperature sensors that give the same or similar temperatureinformation. During normal operations this temperature would be between800° C. to 1200° C., typically 1120° C.

(2) A back end temperature (Tback). This temperature is measured nearthe feed end of the kiln, or at a point in the kiln plant that isindicative of the exit air temperature from the kiln plant and which isthus indicative of the quality of heat transfer that has taken placewith the kiln plant. For the subject plant it is optimally the toptemperature of the pre-heater, as selecting the measurement near thekiln feed end ignores the heat transfer that takes place in thepre-heater or pre-calciner. Again this measurement can be a derivedmeasurement of a number of other temperature measurements. During normaloperations this value is between 250° C. to 350° C., typically 305° C.

(3) O₂ (%). This is a measurement of the oxygen concentration in thekiln, and is indicative of the combustion process in the kiln. Duringnormal operations the value is between 2 to 5%, typically 4%.

(4) NOX (ppm). This is the concentration of NOX in the air existing inthe kiln and is produced in the flame, and is hence indicative of flametemperature. This measurement value could be between 300 to 2000 ppm,and is typically 700 ppm. Due to problems with calibration the relativevalue of this measurement is more important than its absolute value.

(5) CO (%). This is the concentration of carbon monoxide or CO in theexit air from the kiln. A typical value is about 0.03%. This controlledvariable is very important as if it is too high, there can be a dangerof explosions in the precipitator, and thus this CV has to be monitoredvery carefully.

(6) It is envisaged that the SOX concentration could also be used as acontrolled variable.

(7) Manipulated variables (MV) or disturbance variables (DV)

(8) Total coal flow (Ctot). This is the total mass flow of coal beingpumped into the kiln from both the fire-end and the feed-end, typically10 to 12 tons per hour (tph) for the subject kiln plant.

(9) % Coal to back (% Cback). This is the percentage of the total coalbeing pumped to the feed or back end of the kiln. The percentage rangesfrom 5 to 15%, typically 12%, and is dependent on the type of kiln.

(10) Kiln Speed (Spd). The rotational speed of the kiln which is 1 to 3rpm, typically 1.6 rpm. Kiln feed or raw meal feed is linked to kilnspeed and is typically in the range 50 to 400 tons/h or more.

(11) Grate Speed (Grate). Grate speed is the speed of the first gratemotor on the grate cooler. This is expressed as a percentage of maximumgrate speed.

(12) Cooler Air Flow (FanC). A derived air flow figure measured in m³/hrwhich could be either the sum or average of a number air flow fansystems pumping air into the cooler. The value for the subject plant isthe total of three air fan flow rates, which are all under PID control.Should the PID control of any one fall away, then the system can beadapted to disregard that fan and use any of the others. These air flowsare three of approximately 20 fans located in the cooler, and are thoseconsidered by the plant personnel to be the most influential in terms ofcooling.

(13) ID fan speed (FanID). Relative speed of the ID fan to its maximumspeed. The ID fan is normally a variable speed fan. Should the ID fannot be a variable speed fan, then the damper position of the ID fandamper can be used, i.e. this variable has to be something that providesdirect control of the amount of air being drawn into the kiln.

The MV's and CV's are not necessarily limited to the above and can beextended, or can be made smaller. For example, on some kilns there is noback end firing, and some plants have a fixed speed ID-fan, so theID-fan damper setting is used. Another example where a subset is used iswhere the plant does not have all the gas analyzers available. When, forexample, the NOX reading is not available, the controller cells relatedto this can be made “inactive” by switching off these cells so that thisCV is effectively removed from the various optimizing equations that arebeing solved. This can be done by setting the weighting factors for NOXto zero.

The contents of each cell matrix in FIG. 6 is a mathematical descriptionof the interaction of each CV with the appropriate MV or DV, e.g. howthe hood temperature will respond to a change in the total coal input.These descriptions are currently time-based descriptions, and arederived either through first principles, or automatically using thecontroller or other “offline toolkit”, where the characteristic responseis derived automatically from data captured from the plant. There areother means for specifying the characteristic response of a particularvariable to another, and this could be in the “S” or frequency domains,for example. These response descriptions are typically either first orsecond order responses, with time delays. The gains of the responses arederived either automatically using the controller toolkit, or can bespecified manually. Each cell has a time response curve, the magnitudeof the response i.e. the starting point and the final steady statevalue, being the gain.

The gain value to any cell could be constant, linear, or even a functionof one or more other variables.

Each one of the 5 controlled variables has a relationship with each oneof the 6 manipulated variables, making the controller a multi-variablecontroller. If one manipulated variable is moved, say, total coal, itwill have an effect on each of the 5 CV's. The aim of the controller isto enable one to set up a desired value for a target variable, and thento allow the controller to manipulate the manipulated variables in sucha way that they do not move from their targets or are kept within anallowable range. Thus we have a thermodynamic controller, conceptualizedas a 6×5 matrix of interacting manipulated and controlled variables.

Although the above matrix could be implemented using a number ofsoftware packages, the controller on the test plant was implementedusing “Process Perfecter” (trade mark) software by PavilionTechnologies.

In terms of the use of Process Perfecter for the multivariablenon-linear controller, the matrix of 30 relationships has to be defined,so that the controller will know that if a manipulated variable moves ina certain way, then the controlled variables will respond in a known andspecific fashion. This relationship is the single output, single inputresponse of the controlled variable to the manipulated variable.

In order to get the controller to work, a system identificationprocedure has to be performed which identifies the 30 relationships.There are two ways of doing this system identification. These are toperform the identification automatically using the Process Perfecter“auto identification” module, or to use experience and first principlesto derive the relationships. These relationships are then specified as avariety of first or second order responses. If there is no relationshipbetween a manipulated and controlled variable, then we say that themodel representing this pair is zero. The use of plant step tests canalso be used to create step tests data to be used to derive therelationships by any one of the above means.

These relationships are given as a set of parameters, which can begraphed as a time response, where the x-axis is time, and the y-axis isthe magnitude of the response of the controlled variable to a step movein the controlled variable. The model for the time response of eachmanipulated/controlled variable pair is represented by a time responseover a certain time interval and with the plant actually used is 110time intervals, where each interval has been chosen to be 1 minute. Thetime interval of 1 minute was chosen as a result of a study in which theoverall time constants in the kiln processes were observed, and theoperators and experts observed as to how often they make changes to thekiln, and how fast the kiln reacts. Obviously, the number of timeintervals and the length of the time interval could be varied accordingto control requirements, and the process characterisation of thespecific kiln being automated.

In addition, for the purposes of simplification, the variablesassociated with the thermodynamic controller can be split upconceptually into “gas” and “thermodynamic” variables.

Gas states or variables refer to the gas pressures, air flows, fanspeeds and gas composition. Thermodynamic states refer to thetemperatures, and mechanical states of the kiln, which are indicatedprimarily by temperatures, mass flows, kiln speed, and kiln torque whichis indicated in turn by kiln main drive current. The reason for thisclassification is that the gas dynamics are faster than the otherthermodynamic responses and thus some of the dynamics are easier tounderstand in this regard.

All the variables are in effect thermodynamic variables.

The kiln main drive current is also an indicator of the thermodynamicstate and the clinkering process within the kiln. The main drive currentis proportional to the torque required to drive the kiln, which is inturn indicative of the amount of material and clinker coating within thekiln, as well as being indicative of how the coating and brick lining isdistributed around the inner shell of the kiln. Thus, the kiln maindrive current can be used as a disturbance variable in the thermodynamiccontroller. In the prototype system, it was effectively taken intoaccount as an unmeasured disturbance, but it can also be taken intoaccount as a measured disturbance.

The gains of the prototype thermodynamic controller responses aresummarized in the gains matrix.

The grey cells indicate that, for this particular version of thecontroller, no model has been included for the following possiblereasons:

(a) Knowledge and experience, step tests, etc, have shown that there isno relationship between the specific controlled and manipulatedvariable, or that the relationship is so weak that, leaving it out hasno appreciable effect on the kiln operation.

(b) Experience and testing of the controller has shown that there areconflicting moves that are made by the controller when the model in thatarea is active, thus causing oscillation, or instabilities in the closedloop control of the kiln plant. An example is the use of both the totalcoal flow and the ID-fan to control hood temperature.

Once these matrix elemental responses have been specified they can bechanged or modified as more knowledge or information comes to light,thus allowing for incremental or quantum improvements to the controlleras experience is obtained. In addition, the controller can be expandedor contracted as circumstances arise. For example a small controller(5×4) may be derived for a kiln plant that uses simple planetarycoolers. However, if the plant is modified to use grate coolers, thenthe control matrix can be expanded to include more CV's and MV's asapplicable, for example to a (6×4) matrix.

It must be noted that the absolute values of the various variables arenot as important as the relative values of the various controlled andmanipulated variables to each other. It is thus important that thesevariables maintain these relative gains.

In the prototype controller, monitoring of the temperature profile wasdone using one temperature reading indicative of the temperaturedynamics in the vicinity of the hood, as well as one temperature readingindicating the temperature either at the end of the kiln plant orsomewhere in the middle. As can be seen from the control matrix, thecoal or fuel being fed into the kiln is the primary “handle” used tocontrol the temperature profile. This requires controlling the mass flowof coal being fed to the firing end of the kiln.

Control of the combustion process in the kiln is done by monitoring theO₂, CO and NOX gas states of the kiln. The O₂ and CO concentrations givean indication of how well the combustion process is taking place. If theCO is too high it means that there is possibly not enough O₂, etc. NOXis indicative of the actual flame temperature, and this could also beindicated by a pyrometer in the feed end of the kiln, as well as theactual hood temperature itself. In some cases the sensor instruments maynot be available, and the controller is thus tuned or configured atruntime to ignore the particular reading and not take that particulargas CV into account.

The primary means for controlling the combustion process is to controlthe amount of air being drawn through the kiln. This is done primarilyby controlling the ID-fan which, if it is a variable speed fan, is doneby controlling its speed. There are usually air valves or dampers eitherupstream or down stream of the ID)-Fan, and these may also be used tocontrol the kiln air flow. These dampers must be used if the ID-Fan isnot a variable speed fan but a fixed speed fan.

The grate speed is adjusted to change the depth of the hot clinker onthe grate and hence the amount of air and degree of heat transfer thattakes place between the cooling air and the hot clinker. This will havean important effect on the temperature profile of the kiln.

A grate cooler has at least one (typically two) important associatedmanipulated variables (MV) which are used to control the secondary airflow from the cooler to the kiln. This MV is used primarily as a coolinghandle on the kiln, as opposed to the coal which is used as a heatinghandle.

Air flow can also be controlled by varying the primary air feed into thekiln, which can be part of the coal firing system, as the coal isusually fed into the kiln pneumatically. Alternatively, this primary airfeed may be separate from the kiln pneumatic system itself. However,this air flow is in general not really used as its contribution to thetotal air flow may be almost insignificant.

The above control of the cooler is changed depending on the particularcooler and controller that may be supplied with the cooler. Thus thecontrol is over and above other independent control loops that areactive in the cooler such as those effecting pressure control andworking to create various zones or pockets of air within the cooleritself.

The kiln speed is directly coupled to the raw meal feed rate via theplant control system. This coupling is a direct ratio controller whichallows the operator to select a production rate, and the control systemwill then automatically select the correct kiln speed to raw meal feedrate ratio, to allow for correct bed depth control in the kiln. The beddepth or the level of material in the kiln has an optimal level forcorrect and optimal clinker production.

On the test site, the kiln speed was configured as a disturbancevariable (DV) which is set independently by the operator to meet certainproduction goals, and which the kiln controller automatically takes intoaccount due to the fact that there exist cross couplings in the controlmatrix linking the kiln speed to various other controlled andmanipulated variables. This allows the operator to speed up theproduction and have the system automatically take care of the resultingthermodynamic imbalances which will result due to the increase ordecrease in kiln speed.

In a variation of the control scheme, the kiln speed and feed rate couldbe decoupled and treated as separate MV's.

As can be seen from the above, the primary means of thermodynamiccontrol of the kiln is through the adjustment of the following equipmentand settings: ID fan speed, the speed of the kiln, the flow rate of coalbeing fed into the kiln at both ends, and in the case of only certaincoolers the speed of the coolers, and the air mass flow rate into thecooler. There are other items of equipment that may be used, but theseare dependent on the actual physical design of the plant, and aregenerally set to a certain fixed state and left to operate in a constantmode.

Most kiln plants today have a number of base level controllers whichperform some form of automatic control. The base layer controllers arebased on industry standard off-the-shelf controllers which implement PID(Proportional, Integral and Derivative) controllers. In the subjectplant in particular these controllers in general ensure that:

The weight feeders deliver according to desired setpoints;

The kiln and ID-fans maintain a desired speed;

The cooler fans deliver the correct amount of air for a given setpoint.

(There are about 20 PID's on the cooler at the subject plant);

The kiln hood pressure is maintained at a given value. This is done byadjusting the speed of the cooler exhaust fan; and

The cooler speed remains on target.

All the setpoints for these PID controllers are usually set by theoperator, and adjusted on a continuous basis by the operator inaccordance with production goals.

In order for the thermodynamic controller to be integrated into theplant and to operate it must be fed all relevant variables in the plant,and must also be capable of sending variable control instructions backto the plant. The controller is thus connected to the plant via theplant control systems, which in the present case are based on a controlproduct called “KICS” (Knowledge Based and Intelligent Control System)produced by Business Execution systems and Technology (SA) which in turnis based on the real time expert system “G2” from Gensym Corporation.The KICS control system effectively provides all SCADA functionality tothe total cement plant.

The thermodynamic controller of the invention does not necessarily needthe KICS or G2 systems, but can in principle be attached to any cementplant via whatever control system is installed on that plant. Thesecontrol systems can be of any type or form but should satisfy a minimumspecification in terms of control, connectivity, communications, andpossibly pre- and post-processing of data, so that the data processingis easier to configure and set up.

The prototype system also uses another layer with the G2 system toprovide a special custom process control wrapper around the Perfectercontroller. This wrapper is an intelligent real time expert system thathas certain rules and heuristics programmed in which allow for thecorrect operations of the thermodynamic controller. The expert systemprovides various filtering, spike rejection, and other signalconditioning and processing capabilities specifically developed to copewith the peculiarities of the prototype plant, as well as the specialrequirements of the thermodynamic controller.

In particular the expert system provides the means by which the purgingand calibration spikes in the gas analyzers used to provide CO and O2measurements can be rejected. These filters are special moving minimumfilters that are also coupled moving and first order exponentialfilters.

The expert system also provides rules by means of which the correct or“best” analyzer is selected. In addition the expert system also providesthe means by which various hard, fuzzy and/or maximum rate of changeconstraints on the Process Perfecter controller can be changed in realtime due to varying plant conditions and production requirements.

The process control system for the subject plant consists of a SiemensS5 PLC system that interfaces directly with the cement and kiln plant.The system performs all analog and digital I/O, safety interlocks, PIDcontrol, and various other high speed processing. This system alsoprovides full control of all motors on the plant via motor controlcenters (MCC's) located at the relevant points. The communicationbetween the PLC's and KICS is performed via a SETCIM real time database.The overall structure of the plant control system is shown in FIG. 7.

From FIG. 7 it can be seen that the control system consists of 4 primaryKICS control servers, with server terminals or operator screens. Threeof the servers are capable of accessing or controlling the whole plant,while the fourth is specifically reserved to serve the cement loadout,packing and dispatching facilities. The KICS system is connected to thePLC via two SETCIM real time databases, through Aspentech H1 drivermodules as well as KBE G2/SETCIM communication bridges.

All the KICS systems run on Hewlett Packard HP 9000 RISC computers,where the operating systems is HP UNIX, 10.20 or higher. This includesthe SETCIM real-time databases, and communications modules which allexecute on HP UNIX.

The controller connects to the control system via one of the three mainplant KICS servers. Because each server has total plant access it doesnot matter which server is used, or that only one server is connected toa computer running Process Perfecter software. Process Perfecter itselfexecutes on a HP Pentium II PC computer running Microsoft Windows NT4.0.

The overall architecture of the combined control system is shown in FIG.8, from which it can also be seen that the Process Perfecter softwareconnects through a communication system called iDX to talk to the KICScontrol server. iDX is a multi-protocol hub provided by BusinessExecution systems and Technology (SA) (Pty) Ltd which allows G2 to talkto Process Perfecter via the Process Perfecter OPC communication module.

Thus the thermodynamic controller of the present invention provides acontroller which is placed in a closed loop with the cement kiln plant.The controller utilises the existing plant control system, to which itis connected by a communications layer, and is a model-based,multivariable predictive controller implemented in software.

A second aspect of the invention is directed towards the more directcontrol of the quality of the clinker.

The quality of the clinker is determined by analyzing its chemicalcomposition. This is done in the laboratory, where samples of clinkerare analyzed on a regular basis using various manual and/or automatedprocedures and equipment. Depending on how modem the plant is and theinvestment in the laboratory and sampling systems, each plant willmeasure a subset of all possible quality measurements available.

At the plant used to test the invention, the primary qualitymeasurements of clinker used are the clinker free-lime FCAO, the LimeSaturation factor LSF, and clinker 3CaO.SiO₂ (i.e. the amount oftri-calcium silicate in the clinker). Other control parameters thatcould be used and measured include liter-weights, 2CaO.SiO₂, A2F etc.

The free-lime figure is important as it indicates how “hard” the clinkerhas been burnt (ie. the extent to which the clinker has been overburnt),and how much free-lime is left in the clinker after the clinkeringprocess. Too high a free-lime content where the burning has been “soft”(underburnt), is undesirable, as free-lime decreases or degrades thequality of cement. 3CaO.SiO₂ is one of the main final constituents ofclinker. There is an optimal range of values of 3CaO.SiO₂, and a maximumvalue of free-lime which should be maintained at all times. The optimalrange of 3CaO.SiO₂ is typically between about 55% and 75%.

Although zero free-lime would be ideal, the fuel costs associated withheating up the clinker to the degree whereby zero free-lime is achievedare too high, and thus the usual objective is to strive to balancefree-lime at just below 1.5%. This gives the minimum required quality atpossibly the lowest production cost, i.e. with minimum coal injection.The optimum point of 1.5% free-lime is not necessarily universal and mayvary according to local plant design and cement production requirements.

Burning too “hard” to drive the free-lime figure very low has anotherdetrimental affect on the clinker, which becomes extremely hard anddifficult to crush, increasing downstream crushing costs, and henceincreasing the cost of cement production.

The primary control objective of any kiln control system irrespective ofwhether it is automated or not, is to control the quality of clinkerwhilst maintaining production throughput and kiln stability. Qualitycontrol is the control of clinker 3CaO.SiO₂ and clinker free-lime. Thiscontrol involves achieving a desired setpoint for clinker 3CaO.SiO₂ andfree-lime, as well as the minimization of the quality parameterdeviations. The control of the clinker quality must be done whilst atthe same time maintaining mechanical safety, throughput and minimizingcosts. However, there is presently no automated control of 3CaO.SiO₂ orfree-lime. Operators control manually to free-lime targets or otherparameters close to free-lime.

The manual control that does exist with respect to the 3CaO.SiO₂ orfree-lime is the rejection of material that does not conform to requiredstandards, as well as the subsequent adjustment of the thermodynamicstate of the kiln in response to this problem. This material rejectionoccurs if, for example, free-lime is above about 2%. The ideal is tohave free-lime between 1.0 and 1.5%.

Because the automatic or direct control of free-lime is manual,operators bum either harder or softer to either decrease or increase theamount of free-lime in clinker, respectively. There are also othersubsequent manual procedures implemented by operators of the plant tocompensate for raw meal feed problems etc, which contribute to highfree-lime levels to bring free-lime and 3CaO.SiO₂ back into the regimewhere it is acceptable in terms of clinker and cement quality control.

The difficulty with this manual control of either free-lime or 3CaO.SiO₂is that there are many unmeasured and measured disturbances in the kiln,for example, a measured disturbance in the quality of the incoming rawmeal, in terms of actual values and deviations.

Thus, the second aspect of the present invention attempts to control thekiln plant in a multi-layered manner where two or more controlobjectives are always being evaluated. The first or lower level controltask is the thermodynamic control of the kiln, as described above. Theobjective of this task is to maintain the kiln in a thermodynamic statethat enables the production of clinker at the right quality, and atrequired production rates, within various costs and mechanical andprocess constraints. The next layer of control is the adjustment of thethermodynamic state on the basis of the desired free-lime content of theclinker. The next layer of control is the adjustment of the free-limesetpoint to ensure that the 3CaO.SiO₂ content of the clinker and hencethe cement is within specification.

It will be appreciated that 2CaO.SiO₂ could also be considered animportant measure of clinker quality. As such, the 2CaO.SiO₂ content ofthe clinker could be controlled, as could any other important chemicalproperty.

The preferred embodiment of the controller thus provides three layers ofcontrol, namely an inner thermodynamic controller, an intermediate layerfree-lime controller and an outer layer 3CaO.SiO₂ controller. Theimplementation of the free-lime controller is optional and depends onwhether measurements of the free-lime are available in the plant controlsystem. In some cases this second level controller may be a“liter-weight” controller rather than a free-lime controller.

The liter-weight is the manual measurement of the mass of clinkerfilling a liter container. The clinker used in this measurement ispre-screened to a certain size fraction. Thus this measurement gives ameasure of the reactivity of the clinker, or surface area per unit massof clinker. It is a manual alternative to get a good indication of howwell the clinker is being manufactured, and it's chemical andmetallurgical properties. The free-lime controller can be replaced witha “liter-weight” controller, where the thermodynamics of the kiln arecontrolled in a manner similar to the free-lime controller to maintain acertain “liter-weight” specification.

In the light of the above, the controller can be generalized as being atwo layered controller with the lower level being a thermodynamiccontroller, and the higher level being a quality controller, with thequality controller being capable of being divided up further into two ormore layered controllers, namely the intermediate free-lime controllerand the outer 3CaO.SiO₂ controller.

A first embodiment of the prototype three-tier or three layer controlleris shown schematically in FIG. 9. As there are no direct control actionsthat can be taken on the plant to control clinker 3CaO.SiO₂ andfree-lime, control of these quality parameters is indirect, and is doneby adjusting the thermodynamic and operating state of the kiln inresponse to the properties of the raw meal and fuel being fed into thekiln, as well as the emerging clinker properties.

The higher level controllers only deal with trying to maintain qualitywhich is shown to be a fairly slow moving dynamic, while productionoptimisation and the other issues are dealt with in the lower levelthermodynamic controller 46.

In the hierarchical structure of the controller the thermodynamiccontroller 46 is given setpoints for maintaining the thermodynamic stateas determined by the higher level controller so as to maintainfree-lime, or 3CaO.SiO₂ quality levels. In the first illustratedembodiment of the invention, this higher level controller is firstly the3CaO.SiO₂ controller, which then feeds the free-lime controller with adynamic free-lime target, which is the required free-lime as issued bythe 3CaO.SiO₂ Controller to steer the 3CaO.SiO₂ back to target. Therelationship between 3CaO.SiO₂ and free-lime will be described below.

The 3CaO.SiO₂ controller 42 accepts a target value for 3CaO.SiO₂, aswell as the current 3CaO.SiO₂ as measured on the plant by thelaboratories.

The current 3CaO.SiO₂ value is fed back to the 3CaO.SiO₂ controller 42using a first feedback loop 48. The controller then calculates afree-lime value that will bring the current 3CaO.SiO₂ process value tothe target value, whilst taking into account other factors which affect3CaO.SiO₂.

In this controller the 3CaO.SiO₂ is the controlled variable, and thefree-lime is the manipulated variable.

The basic relationship between 3CaO.SiO₂ and free-lime has been derivedin a number of ways which are used to derive the top level 3CaO.SiO₂controller relationship.

The first is an empirical mathematical relationship where

3CaO.SiO₂=f(FcaO,LSF,SRALM) and =f{CaO,SiO₂,Al₂O₃,TiO₂,Fe₂O₃,Mn₂O₃,SO₃}

Where FCaO is free-lime

SR is the silica ratio

LSF is the lime saturation factor

ALM is the alumina modulus

and the others are various chemical oxides, etc

The various chemical analyses are calculated from X-ray analysis of thevarious samples taken in the plant on raw meal and clinker, and thenused to calculate the values of SR, ALM, LSF, etc. SR and LSF, which arefed back to the 3CaO.SiO₂ controller together with the measured3CaO.SiO₂ content, as is shown at 49.

The second relationship is a neural network relationship where variousplant variables and 3CaO.SiO₂ are the inputs and the output is3CaO.SiO₂. This neural network is optional and is used to provideestimates for 3CaO.SiO₂ during the sample times of the actual 3CaO.SiO₂laboratory measurements. If the neural network is not possible due tolack of various inputs, the neural network can be replaced with a sampleand hold or predictor type estimator of 3CaO.SiO₂.

The third relationship uses a basic equation$\frac{{FCaO}}{{C_{3}}S} = \frac{{FCaO}_{target} - {FCaO}_{setpoint}}{{C_{s}S_{target}} - {C_{s}S_{labt}}}$

This relationship is shown implemented in the block diagram of FIG. 10.

In the free-lime controller 44, the measured free-lime from the plant isfed back to the controller 44 using a second feedback loop 50, and iscompared to the free-lime setpoint or free-lime target. The free-limecontroller 44 then determines what the required thermodynamic targets 52should be in order to meet the target free-lime, i.e. to reduce thefree-lime error to zero.

There are a variety of definitions of the error to which the ProcessPerfecter controller is to operate. The first and obvious type is thedeviation or difference between actual and desired controlled variablessuch as free-lime. However, the error could also be the amount by whichthe free-lime value intrudes out of an allowed regime of operation intoa fuzzy or hard limit area of undesirable operation.

The thermodynamic targets used in the prototype as the output of thefree-lime controller, and constituting the controlled variables, are asfollows:

Hood temperature

Backend temperature

Required CO level

Required NOX level

Required O₂ level

The free-lime controller 44 gets its actual plant measurement or currentprocess value of free-lime from one of two sources.

(1) The first is a neural network based free-lime predictor or virtualon-line analyser (VOA) 54, which predicts the current instantaneousfree-lime value from various plant process parameters, including theplant quality lab values 40.

(2) If for some reason the free-lime predictor or virtual on-lineanalyser (VOA) does not work, then the free-lime controller receives itsfree-lime value from the actual plant laboratory quality control system40 and 56. This is fed in automatically from the plants' quality controlsystems via the plant control system, or is fed in by hand into theplant control system, if the communications link between the plantquality control system and the plant control system fails.

The free-lime controller 44 examines both the free-lime target and theactual free-lime, and then based on this error outputs calculates whatthe required setpoints for the thermodynamic variables are. Thefree-lime controller calculates a profile of moves into the future thatit will perform to the manipulated variables in order to reduce theerror to zero, or close to zero. The shape of this profile of moves isdependent upon:

(a) the models between the manipulated and controlled variables, and

(b) the tuning parameters that are given to the controller in respect ofeach controlled variable/manipulated variable pair.

The model of how manipulated variables influence free-lime is stored ina matrix of relationships in the controller software. Theserelationships are typically first or second order time responses, whichare the responses of the free-lime to step changes with the value of thethermodynamic manipulated variables. In other words how:

(a) Free-lime changes in responses to a step change in the percentagecoal to be fed to the back end of the kiln;

(b) Free-lime changes in response to a step change in hood temperature;

(c) Free-lime changes in response to a step change in backendtemperature;

(d) Free-lime changes in response to a step change in CO levels;

(e) Free-lime changes in response to a step change in NOX levels.

The models of these step response relationships were established bycement experts and operators on the plant, as well as by examining therelationship between captured data of free-lime and changes in thesevariables. These data were captured from normal running operations thatwere stored in a real time database, as well as from explicit step teststhat were taken during the time of the project step test phase.

The five responses for the free-lime controller are all second ordercritically damped responses.

For example, a change in Thood from 1000 to 1500 degrees i.e. 500° C.would induce a change of approximately 0.1% in free-lime FCaO. Thisamounts to a gain of 0.0002 and because the increase in temperaturedrops the free-lime, the gain is negative. The absolute values are notimportant, but rather the relative values. To make the absolute valueslook more realistic when simulating, one adjusts an offset to bring theabsolute values in line with real plant values.

At present the free-lime controller assumes that the gains of each ofthe four responses are constant, i.e. that the gains are linear.However, these gains may be made to be functions of the plant statesthemselves, i.e. to be non-constant. In this case the gains will changedepending on the plant conditions. This situation (when the gains are afunction of the plant state) is the non-linearity that is generallydisplayed by all processes.

So for example, assuming that a 1° change in hood temperature gives a−0.002% change in free-lime, if the hood temperature increases from1000° C. to 1050° C. this value may change by −0.1%.

The free-lime gains used in the prototype free-lime controller were asfollows:

Thood gain: −0.002

Tback gain: −0.001

CO gain: 10

NOX gain: −0.004

As the current system thermodynamic controller 46 has five controlledvariables, there is the potential to set or determine all five of thesecontrolled variables, using the quality controller. Thus the controllerfor free-lime only is a 1×5 or 1×N controller. Should a plant desire tocontrol clinker free-lime and clinker LSF, for example, at this levelthen this controller will become a 2×5 controller.

In the prototype kiln, four of the controlled variable targets came fromthe free-lime controller, while the O2 target was input manually by anoperator. Thus it is possible to add and remove various controller andmanipulator variables depending on the particular setup of the kiln.

Another possible embodiment of the free-lime or quality controller, isthat the controller can be implemented either as a combination ofmathematical relationships as given above or by heuristic relationshipsor rules. This is a possible alternative that uses the expert systemshell or other software to implement the relationships between thefree-lime and the inputs to the thermodynamic controller.

This use of rules with the controller is an option that is determined bythe type of kiln, control infrastructure, and the degree of automationrequired and associated complexity that comes from the degree of benefitor value added. This way of implementing the quality controller is justan alternative in terms of technology chosen, and is used to illustratethe relative independence of the controller from the type of technology.

FIG. 11 illustrates an alternative embodiment of the second aspect ofthe invention. In this embodiment, the quality controller is partiallyparallel to and partially cascaded to the thermodynamic controller 46.

The output of the free-lime controller 44 is both actual plantmanipulated variables and possibly one or more thermodynamic variables.In a first mode of operation, the free-lime controller outputs thesetpoint of the percentage coal to the back end of the kiln, while theother setpoints are controlled by the thermodynamic controller 44. Thusthe setup is a partially cascaded configuration in that the 3CaO.SiO₂controller 42 is cascaded to the free-lime controller 44, and that thesetwo together operate in parallel with the thermodynamic controller 46.

Although the manipulated variable, percentage coal to the back (%Cback), is controlled by the free-lime controller, its effects as adisturbance to the thermodynamic controller is still taken in toaccount, and at any time it can be activated so that the thermodynamiccontroller can control the percentage back end coal as well.

Dotted lines 58 in the Figure illustrate the built-in possibility ofthis embodiment being configured to the same layout as the firstembodiment, in which the output of the free-lime controller 44 will beinputted to the thermodynamic controller 46 only. This is to allow thecontroller to be adapted for use on kilns with no coal feed to the backend of the kiln. Thus the indirect method of deriving a thermodynamicoutput from the free-lime controller in a cascaded configuration tocontrol the thermodynamic controller will be the configuration used inthis case.

When the thermodynamic controller and free-lime controller are workingin tandem, the goal is to produce good quality clinker by thecontrollers together creating a stable and correct thermodynamicenvironment in which the various clinkering processes can take place.The temperature profile is controlled by manipulating two profilevariables, i.e. the profile level or height and the slope of thetemperature profile. The two corresponding handles are the hoodtemperature and the back end coal feed, which are controlled by thethermodynamic and free-lime controller respectively.

FIG. 12 illustrates a third embodiment of the second aspect of theinvention.

This embodiment focuses on the fact that the primary quality parameterof clinker is the 3CaO.SiO₂ component. Thus this embodiment attempts tocontrol 3CaO.SiO₂ whilst allowing the free-lime value to range freelywithin its constraints, ie. below the maximum level of 1.5%, forexample.

The main disturbances that influence the 3CaO.SiO₂ content of theclinker are the thermodynamic conditions in the kiln, the properties ofthe incoming raw material and the properties of the coal or fuel used inthe kiln. Thus the 3CaO.SiO₂ controller 42 receives a setpoint for the3CaO.SiO₂ value together with measurements indicative of variousproperties of the incoming raw material and fuel used in the kiln. The3CaO.SiO₂ controller 42 then outputs a target to the thermodynamiccontroller 46. In the present embodiment, this target is the target hoodtemperature Thood, but other temperature readings could equally be used,such as the NOX reading or one or more back end temperature readings.

The 3CaO.SiO₂ controller 42 may also input the free-lime target to thefree-lime controller 44, as indicated by dotted line 60 in the Figure.

The overall controller of the invention is run by executing the ProcessPerfecter software and all relevant communications modules orexecutables on the designated computer. When these executables run,bi-directional communications are automatically established with thekiln plant DCS, and the Process Perfecter as well as on-line neuralnetwork modules start executing immediately. However, initially orunless otherwise specified, the controller does not run the plant, andthe operators are still in control. This is because MV setpointstransmitted from the controller to the control system are nottransmitted to the field, until appropriate software switches in the DCSor SCADA system are closed. Full closed loop control is enabled whenthese setpoint switches in the control system are switched on.

When the controller is in control of the kiln plant, the operator canmonitor, adjust, fine tune and change the controller's behaviour. Thisalso includes enabling or disabling the inclusion of the higher levelcontrollers. This control is performed either via Process Perfecter'sown GUI's or via GUI's engineered in the control system. The finalengineered solution allows explicit control and adjustment of thecontrol system goals, constraints and tuning parameters both from theDCS/KICS GUI's and the Process Perfecter GUI. The use of external GUI'sto perform this control is facilitated by the availability of theProcess Perfecter GUI parameters via the PDI to the external controlsystem.

The controller's functions are made up of the basic functionality of theProcess Perfecter software as well as additional functionalityengineered into the external control system. The Process PerfecterGUIs's provide the following control functionality with respect to thecontroller:

(a) Control of each SISO controller cell from the total control matrix,which can be switched on or off. This allows for either full orselective control of the kiln plant. Thus the thermodynamic controllercan be used for example to control the cooler only, or the kilntemperatures only.

(b) The operator can change targets or setpoints for any controlled ormanipulated variable in any level controller. Obviously if a higherlevel controller is active in the loop then individual settings oftargets in lower level controller will be overwritten by the higherlevel controller. Setting a manipulated variable to a certain targetimplies that the variable can not be used for control, and thisconstrains what the controller can do to control the kiln.

(c) The speed of the kiln has been made a disturbance variable, and isthus controlled by the operators. The speed of the kiln is directlylinked to the raw meal feed, thus changing the speed of the kiln,changes the feed rate of raw meal into the kiln, and hence is a majorknown disturbance into the kiln. The philosophy of controllingthroughput via the kiln speed is used because the raw meal feed is aslave to the speed of the kiln i.e. the raw meal feed is in linearcascade to the speed of the kiln. Thus in other kilns the controller mayuse either kiln speed or raw meal feed or both.

(d) The operators can set various hard constraints onto every MV or CV.These hard constraints allow for the operating range of the plant to beset in terms of the minimum or maximum excursions the manipulatedvariables can take in terms of control. Setting hard constraints on thecontrolled variables, does not mean that they cannot be violated, butdoes mean that severe penalties will be incurred should they violated,and the controller will bring all effort into reducing these hardconstraint violations.

(e) Soft or fuzzy constraints. All variables in the controller's matrixcan be given soft constraints. Violation of the soft constraints meansthat the variable will incur an ever increasing cost penalty with regardto its violation. This implies that if necessary the controller willviolate these constraints, but will eventually endeavor to minimizethese constraints on the basis of all other constraints within thesystem, hence allow for optimizing of the kiln plant in terms ofunstable or bad excursions of various process variables.

(f) Priorities and weights. All constraints, targets, etc can havevarious weights, or costs, that allow one to tune the kiln plant so thatvarious process deviations take priority over others, hence ensuringthat correct control action will be taken on the kiln. Because thepriorities etc, can be changed at run time, this implies that the tuningof the optimization of each controller can be adjusted by eitheroperator of the plant control system providing for adaptive optimisationof the controller according various criteria.

(g) Frustums. Frustums are similar to soft or fuzzy constraints, exceptthat the penalties incurred become more severe with time. This meansthat the controller endeavors at all times to bring any target withinthe frustum so as to minimize process deviations with time into thefuture.

(h) Rate of change constraints. Each process variable (in particularManipulated Variables) has a maximum rate of change setting that can beset for either up or down movements. This provides a safety mechanism inthat various control actions over time can be limited or constrained soas not introduce too severe a change into the kiln hence making itunstable.

Due to the graphic user interface features of the Process Perfectersoftware, the controller provides a full multivariable view of the plantand its status. These views consist of real time plot and trends thatdisplay the present and past history of all variables used in thecontroller. These views also incorporate predictions of the futurebehavior of the cement kiln plant. Thus the operator can see whathis/her or the controller's effect on the plant will be into the futurefor a time horizon that can be set by the user.

Various control and optimisation mechanisms can be introduced into thecontroller that fulfill the operational, safety, and business objectivesof the kiln plant. For example, to optimize the kiln in terms of costsincurred due to production, the coal settings will be set so that toohigh a coal usage will incur a high penalty. Thus if the use of the IDfan or another manipulated variable is set to a lower cost or prioritythan coal, the controller will make more use of secondary air from thecooler in a bid to maintain the high temperature required, instead ofputting more coal into the kiln. This type of control strategy willobviously be offset or traded against the need to maintain hightemperatures, but not too high, and thus the controller will evaluatethese needs against for example the NOX targets, which are indicative ofthe flame temperature.

The benefits of the controller run on the test plant were numerous.

Variations on the hood temperature were reduced markedly compared toprior operation of the kiln, especially in the presence of majordisturbances into the kiln plant system such as “poking” and “coatingdrops”. In addition the controller maintained control through theseperiods of disturbance, whereas prior art controllers have not been ableto cope with the disturbance and have relinquished control of the kilnto special sub-routines designed only to cope with such situations.

Furthermore, the controller has coped well with off-specification feedmaterial which has high or low LSF in the incoming raw meal feed. HighLSF implies that more energy is required to perform the calciningprocess. This problem is usually temporary and can cause the free-limecontent in the clinker to be high and hence result in off specificationclinker.

Furthermore, the back-end temperature of the kiln under the controllerhas been observed to be steady at optimal temperatures, i.e. at designedlow levels that indicate optimal energy absorption terms of thecalcining and the reduction of heat vented to atmosphere.

The CO levels were maintained within safety levels, and when highs orexcursions occur the controller reacts optimally and safely to bring theCO levels back into the same range.

The controller maintained O₂ levels within the desired operating range.

Sustained higher throughput or production rates have been realized thatare much higher than achieved under operator control.

Use of the controller resulted in a significant improvement in kilnefficiency (ie. the amount of energy consumed per ton of clinker) andhence resulted in significant electrical energy savings.

Use of the controller also resulted in a significant saving inrefractory bricks due to better kiln stability.

Additionally, savings on electricity were also noted.

Clinker free-lime is maintained for longer periods of time within thetarget values and with smaller deviations, and off specification clinkerlevels were much lower than previously experienced. Because of this, theoff specification clinker silo was quickly empty.

The controller had near to 100% up time, and enjoyed operator andmanagement acceptance. The system made fewer demands on the operators,resulting in a need for fewer operators.

All of the above resulted in large cost savings.

Thus it will be appreciated that the present invention provides acontrol system which not only provides an advanced thermodynamiccontroller, but which integrates the quality and thermodynamic controlof a kiln plant.

What is claimed is:
 1. A controller for a kiln plant comprising a kilnhaving a firing end and a back end, wherein fuel is combusted at thefiring end and raw meal is fed into the back end for conversion toclinker in the kiln, the controller comprising: a first temperaturesensor arranged to measure the temperature at or near the firing end ofthe kiln and to generate an output indicative of this temperature; gassensing means arranged to measure the concentration in the kiln of atleast one gas from the group comprising O₂, NOX, SOX and CO and togenerate at least one respective output indicative of the relevant gasconcentration; and control means adapted to receive the output from thefirst temperature sensor and to control the amount of fuel fed to thefiring end of the kiln to maintain the temperature at or near the hoodof the kiln within a predetermined range, the control means beingfurther adapted to receive said at least one output from the gas sensingmeans and to control at least a main impeller of the kiln to maintainthe concentration of said at least one gas within a predetermined range,thereby to maintain a desired thermodynamic profile in the kiln tocontrol and optimise the properties of clinker produced therein.
 2. Acontroller according to claim 1 which further comprises a secondtemperature sensor arranged to measure the temperature at or near theback end of the kiln and to generate an output indicative of thistemperature, the control means being further adapted to receive theoutput from the second temperature sensor and to control the amount offuel fed to the back end of the kiln to maintain the temperature at ornear the back end of the kiln within a predetermined range.
 3. Acontroller according to claim 2 wherein the control means furtherincludes a control matrix which includes values determinative of therelationships between a plurality of plant measurements including thetemperature at or near the hood of the kiln, the temperature at or nearthe back end of the kiln, and O₂, NOX, SOX and CO concentrations, and aplurality of operating parameters including the amount of fuel fed tothe firing end of the kiln, the amount of fuel fed to the back end ofthe kiln, the main impeller speed, the kiln speed, the kiln main drivecurrent, the raw meal feed, the cooler air flow and cooler grate speed.4. Control means for a kiln plant, the control means comprising an outerquality controller cascaded to at least one inner controller, whereinthe outer quality controller comprises a first feedback controller beingadapted to receive a first setpoint input indicating a desired amount of3CaO.SiO₂ and/or 2CaO.SiO₂ to be present in clinker produced by the kilnplant, and a second feedback input indicating the actual amount of3CaO.SiO₂ and/or 2CaO.SiO₂ present in clinker being produced by the kilnplant, the first feedback controller being further adapted to comparethe first setpoint input and the second input and, if the inputs differ,to produce an output to alter a setpoint input to the inner controllerdirectly or indirectly to adjust one or more of the kiln plant'soperating parameters so that the amount of 3CaO.SiO₂ and/or 2CaO.SiO₂ inthe clinker produced by the kiln plant will be substantially equal tothe desired amount of 3CaO.SiO₂ and/or 2CaO.SiO₂ of the clinker. 5.Control means according to claim 4 wherein the inner controller is afree lime controller, wherein the second setpoint input comprises adynamic setpoint for the free lime content of the clinker to the innercontroller, and wherein the inner controller is adapted to receive aninput indicating the actual amount of free lime present in clinker beingproduced by the kiln plant, the inner controller being further adaptedto compare the dynamic setpoint for the free lime content and the actualamount of free lime present and, if these differ, to produce an outputto directly or indirectly alter one or more of the kiln plant'soperating parameters so that the amount of free lime present in theclinker produced by the kiln plant will be substantially equal to thedynamic setpoint for the free lime content.
 6. Control means accordingto claim 5 wherein the control means further includes a thermodynamiccontroller, cascaded to the inner controller, wherein the innercontroller outputs a setpoint for at least one plant measurement to thethermodynamic controller, and wherein the thermodynamic controller isadapted to receive an input from the kiln plant indicating the value ofthe at least one plant measurement, the thermodynamic controller beingfurther adapted to compare the setpoint for the at least one plantmeasurement and the value of the at least one plant measurement and, ifthese differ, to produce an output to alter one or more of the kilnplant's operating parameters.
 7. Control means according to claim 5 orclaim 6 wherein the inner controller is arranged to output a pluralityof dynamic setpoints for a plurality of plant measurements to thethermodynamic controller, the plurality of plant measurementsconstituting controlled variables and being selected from the groupincluding the back end temperature, the hood temperature, the level ofCO, the level of NOX, the level of SOX and the level of O₂.
 8. Controlmeans according to claim 5 wherein the kiln plant's operating parameterscomprise one or more of the group constituting manipulated variablescomprising the total fuel fed to the kiln plant, the percentage fuel fedto the back of the kiln plant, the main impeller speed, the kiln speed,the cooler air flow and the cooler grate speed.
 9. Control meansaccording to claim 8 which is arranged to manipulate the parameters toalter the plant measurements to approach respective setpoints, using acontrol matrix which includes values determinative of the relationshipsbetween the operating parameters and plant measurements.
 10. Controlmeans according to claim 5 wherein the control means further includes athermodynamic controller connected to the kiln plant, wherein thethermodynamic controller is adapted to receive an input from the kilnplant indicating the value of at least one plant measurement, thecontroller being further adapted to compare a setpoint for the at leastone plant measurement and the value of the at least one plantmeasurement and, if these differ, producing an output to alter one ormore of the kiln plant's operating parameters, wherein the at least oneof the kiln plant's operating parameters controlled by the thermodynamiccontroller is different from the one or more operating parameterscontrolled by the free lime controller.
 11. Control means according toclaim 10 wherein the at least one or more of the kiln plant's operatingparameters controlled by the thermodynamic controller comprise at leastone of the group comprising the total coal fed to the kiln, the mainimpeller speed, the kiln speed, the raw meal feed, the cooler air flowand the cooler grate speed and wherein the kiln plant's operatingparameter controlled by the free lime controller is the percentage fuelfed to the back of the kiln.
 12. Control means according to claim 5wherein the inner controller is a thermodynamic controller, wherein thesecond setpoint input from the outer quality controller to thethermodynamic controller is a setpoint for at least one plantmeasurement, and wherein the thermodynamic controller is adapted toreceive an input from the kiln plant indicating the value of the atleast one plant measurement, the thermodynamic controller being furtheradapted to compare the setpoint for the at least one plant measurementand the input indicating the value of the at least one plant measurementand, if these differ, to produce an output to alter one or more of thekiln plant's operating parameters.
 13. Control means according to claim12 wherein the at least one plant measurement is one or more of theplant measurements selected from the group including the back endtemperature, the hood temperature and the level of NOX.
 14. Controlmeans according to claim 13 wherein the selected plant measurement isthe hood temperature.
 15. Control means according to any one of claims12 to 14 wherein the control means include a free lime controllerarranged to receive a setpoint input for the free lime content of theclinker and an input indicating the actual amount of free lime presentin clinker being produced by the kiln plant, the free lime controllerbeing further adapted to compare the setpoint for the free lime contentand the input indicating the actual amount of free lime present and, ifthese differ, to produce an output to directly or indirectly alter oneor more of the kiln plant's operating parameters so that the amount offree lime present in the clinker produced by the kiln plant will besubstantially equal to the setpoint for the free lime content. 16.Control means according to claim 15 wherein the setpoint for the freelime is received from the 3CaO.SiO₂ controller.
 17. Control meansaccording to claim 15 wherein the setpoint for the free lime is manuallyinput by an operator of the controller.
 18. Control means according toclaim 15 wherein the one or more operating parameters controlled by thefree lime controller are different from the one or more operatingparameters controlled by the thermodynamic controller.
 19. Control meansaccording to claim 15 wherein the operating parameter controlled by thefree lime controller is the percentage fuel fed to the back of the kiln.